1. Field of the Invention
The present invention relates to an automatic transmission primarily intended for motor vehicle use, and more particularly, to a solenoid-actuated valve arrangement in the fluid system of an automatic transmission which allows limited operation of the transmission in the absence of electrical power to these valves.
2. Description of Related Art
Generally speaking, land vehicles require three basic components. These components comprise a power plant (such as an internal combustion engine) a power train and wheels. The internal combustion engine produces force by the conversion of the chemical energy in a liquid fuel into the mechanical energy of motion (kinetic energy). The function of the power train is to transmit this resultant force to the wheels to provide movement of the vehicle.
The power train's main component is typically referred to as the "transmission". Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. The vehicle's transmission is also capable of controlling the direction of rotation being applied to the wheels, so that the vehicle may be driven both forward and backward.
A conventional transmission includes a hydrodynamic torque converter to transfer engine torque from the engine crankshaft to a rotatable input member of the transmission through fluid-flow forces. The transmission also includes frictional units which couple the rotating input member to one or more members of a planetary gearset. Other frictional units, typically referred to as brakes, hold members of the planetary gearset stationary during flow of power. These frictional units are usually brake clutch assemblies or band brakes. The drive clutch assemblies can couple the rotating input member of the transmission to the desired elements of the planetary gearsets, while the brakes hold elements of these gearsets stationary. Such transmission systems also typically provide for one or more planetary gearsets in order to provide various ratios of torque and to ensure that the available torque and the respective tractive power demand are matched to each other.
Transmissions are generally referred to as manually actuated or automatic transmissions. Manual transmissions generally include mechanical mechanisms for coupling rotating gears to produce different ratio outputs to the drive wheels.
Automatic transmissions are designed to take automatic control of the frictional units, gear ratio selection and gear shifting. A thorough description of general automatic transmission design principals may be found in "Fundamentals of Automatic Transmissions and Transaxles," Chrysler Corporation Training Manual No. TM-508A. Additional descriptions of automatic transmissions may be found in U.S. Pat. No. 3,631,744, entitled "Hydromatic Transmission," issued Jan. 4, 1972 to Blomquist, et al., and U.S. Pat. No. 4,289,048, entitled "Lock-up System for Torque Converter," issued on Sept. 15, 1981 to Mikel, et al. Each of these patents is hereby incorporated by reference.
In general, the major components featured in such an automatic transmission are: a torque converter as above-mentioned; fluid pressure-operated multi-plate drive or brake clutches and/or brake bands which are connected to the individual elements of the planetary gearsets in order to perform gear shifts without interrupting the tractive power; one-way clutches in conjunction with the frictional units for optimization of power shifts; and transmission controls such as valves for applying and releasing elements to shift the gears (instant of shifting), for enabling power shifting, and for choosing the proper gear (shift point control), dependent on shift-program selection by the driver (selector lever), accelerator position, the engine condition and vehicle speed.
The control system of the automatic transmission is typically hydraulically operated through the use of several valves to direct and regulate the supply of pressure. This hydraulic pressure control will cause either the actuation or deactuation of the respective frictional units for effecting gear changes in the transmission. The valves used in the hydraulic control circuit typically comprise spring-biased spool valves, spring-biased accumulators and ball check valves. Since many of these valves rely upon springs to provide a predetermined amount of force, it will be appreciated that each transmission design represents a finely tuned arrangement of interdependent valve components. While this type of transmission control system has worked well over the years, it does have its limitations. For example, such hydraulically controlled transmissions are generally limited to one or a very small number of engines and vehicle designs. Therefore, considerable cost is incurred by an automobile manufacturer to design, test, build, inventory and repair several different transmission units in order to provide an acceptable broad model line for consumers.
Additionally, it should be appreciated that such hydraulically controlled transmission systems cannot readily adjust themselves in the field to compensate for varying conditions such as normal wear on the components, temperature swings and changes in engine performance over time. While each transmission is designed to operate most efficiently within certain specific tolerances, typical hydraulic control systems are incapable of taking self corrective action on their own to maintain operation of the transmission at peak efficiency.
However, in recent years, a more advanced form of transmission control system has been proposed, which would offer the possibility of enabling the transmission to adapt itself to changing conditions. In this regard, U.S. Pat. No. 3,956,947, issued on May 18, 1976 to Leising, et al., which is hereby incorporated by reference, sets forth a fundamental development in this field. Specifically, this patent discloses an automatic transmission design which features an "adaptive" control system that includes electrically operated solenoid-actuated valves for controlling certain fluid pressures. In accordance with this electric/hydraulic control system, the automatic transmission would be "responsive" to an acceleration factor for controlling the output torque of the transmission during a shifting from one ratio of rotation (between the input and output shafts of the transmission) to another. Specifically, the operation of the solenoid-actuated valves would cause a rotational speed versus time curve of a sensed rotational component of the transmission to substantially follow along a predetermined path during shifting.